When molten aluminum is supplied into the cavity of a mold for aluminum casting, it is likely that an oxide film may form on the surface of the molten aluminum and increase the surface tension of the molten aluminum and lower its fluidity. When an oxide film has formed on the molten aluminum surface, therefore, it is difficult to maintain a good distribution of the molten aluminum.
Accordingly, JP-A-2000-280063 entitled Aluminum Casting Process is, for example, proposed as a casting process making it possible to maintain a good distribution of molten aluminum for aluminum casting. This art will now be described with reference to FIG. 57 hereof.
Nitrogen gas (N2 gas) is first supplied from a nitrogen gas bottle 550 to fill the cavity 552 of a mold 551 for aluminum casting. Then, nitrogen gas is delivered to a storage tank 553 so that a powder of magnesium (Mg powder) in the storage tank 553 may be delivered into a heating oven 555 with nitrogen gas.
The magnesium powder is sublimated in the heating oven 555 and the sublimated magnesium is reacted with nitrogen gas to form a gaseous magnesium-nitrogen compound (Mg3N2).
The magnesium-nitrogen compound is introduced into the cavity 552 of the mold 551 through a pipeline 556 so that the introduced magnesium-nitrogen compound may be deposited on the wall of the cavity 552.
Then, molten aluminum 557 is supplied into the cavity 552. The supplied molten aluminum 557 is reacted with the magnesium-nitrogen compound, so that oxygen may be removed from the oxide on the surface of the molten aluminum 557.
As a result, it is possible to prevent the formation of any oxide film on the surface of the molten aluminum 557 and restrain any increase in the surface tension of the molten aluminum 557. Accordingly, it is possible to maintain a good distribution of the molten aluminum 557 in the cavity 552 and thereby produce an aluminum casting of high quality.
Description will now be made in detail of a step for the formation of the magnesium-nitrogen compound mentioned above and a step for the pouring of the molten aluminum.
Description will first be made of the step for the formation of the magnesium-nitrogen compound. The magnesium powder is sublimated in the heating oven 555 and the sublimated magnesium is reacted with nitrogen gas in the heating oven 555. As the sublimated magnesium is floating in the heating oven 555, nitrogen gas adheres to the whole surfaces of the magnesium and forms the magnesium-nitrogen compound on the whole surfaces.
Reference is now made to FIG. 58 for the description of the step for the pouring of the molten aluminum in the aluminum casting process.
FIG. 58 shows that the molten aluminum 557 has been supplied into the cavity 552 after the deposition of a layer 559 of the magnesium-nitrogen compound on the wall of the cavity 552.
When the molten aluminum 557 has been supplied into the cavity 552, its surface 557a contacts the surface 559a of the magnesium-nitrogen compound layer 559, and oxygen is removed from an oxide 557b formed on the surface 557a of the molten aluminum 557.
The contact of the surface 557a of the molten aluminum 557 with the surface 559a of the magnesium-nitrogen compound layer 559 makes it possible to remove oxygen from the oxide 557b formed on the surface 557a of the molten aluminum 557.
It, therefore, follows that it is sufficient for only the surface 559a of the magnesium-nitrogen compound layer 559 contacted by the surface 557a of the molten aluminum 557 to exist for removing oxygen from the oxide 557b formed on the surface 557a of the molten aluminum 557.
Nitrogen gas, however, adheres to the entire surface of the magnesium, since the formation of the magnesium-nitrogen compound is carried out with magnesium floating in the heating oven 555, as explained with reference to FIG. 57. Accordingly, the magnesium-nitrogen compound is formed on the entire outer surface of the magnesium. The deposition of the magnesium-nitrogen compound on the wall of the cavity 552 forms the magnesium-nitrogen compound layer 559 having a thickness t as shown in FIG. 58.
Thus, an excessive magnesium-nitrogen compound layer 559 is deposited on the wall of the cavity 552, and the formation of the magnesium-nitrogen compound layer 559 takes a long time making it difficult to achieve high productivity.
In addition, the formation of the excessive magnesium-nitrogen compound layer 559 means the use of a large amount of nitrogen gas making it difficult to achieve a reduction of cost.
Moreover, the casting process according to the publication mentioned above is a process that includes the step of filling the cavity 552 with nitrogen gas, while air still remains in the cavity 552, before the step of forming the magnesium-nitrogen compound layer 559 on the wall of the cavity 552.
As a result, it is difficult to have air released smoothly from the cavity 552, and the creation of a nitrogen gas atmosphere in the cavity 552 take a long time making it difficult to achieve high productivity.
There is an aluminum casting having a portion of small thickness, and the known aluminum casting process shown in FIG. 57 may find it difficult to maintain a good distribution of molten aluminum in the cavity when molding an aluminum casting having a portion of small thickness.
Therefore, it is necessary to employ a somewhat prolonged pouring time for molten aluminum in order to ensure a full distribution of the molten aluminum through the whole cavity. Accordingly, the molding of an aluminum casting requires a prolonged cycle time that lowers productivity.